MANUFACTURES.

817

nica*

sses.

^anufac.

tires.

called hematites or bog'-ore. This must be incorporatedwith some agglutinating substance, such as gum, glue,or rosin, to which is sometimes added soap for softeningthe composition ; but instead of reddle other red oxidesof iron, as colcothar of vitriol, &c. are used. These shouldbe selected from those which are the softest to the touch,and of a lively colour, avoiding any mixture of clay,which produces a yellow tint. The ochre thus selectedshould be ground with pure water on a marble slab,as in preparing colours for painters, and moistened onlywith sufficient water to make the stone slide. For pre-paring a large quantity, the reddle must be poundedand sifted through a fine sieve, then diluted with a largequantity of water in a trough, where, after being wellstirred, it should be left for a few minutes to settle, thatthe grosser particles may subside to the bottom. Thewater, strongly impregnated with the finer particles, mustthen be poured off and allowed to settle for twenty-fourhours. When the clear water has been poured off, afine sediment will be obtained, which must be againpounded and washed ; and this process must be repeatedtill the whole is reduced to the utmost fineness. Thegum, glue, or soap necessary for giving to the crayons adue degree of solidity must be separately dissolved, thesolution carefully mixed with pounded ochre or reddle,and the water evaporated by exposure to the sun or to theheat of a gentle fire, taking care frequently to turn thePaste till it has acquired a consistence somewhat harderthan butter. The crayons are then to be formed in themoulds, which may be done two ways: first, by spread-mg out the paste on a board, in which grooves are cutrather broader at the top and round at bottom, and of anylength, size, and depth, according to the proposed bulkof the crayons ; or otherwise, which is the better method,hy forcing the paste through a pipe or funnel of anorifice equal to the size of the crayon. The paste thusformed may be left to dry in a cool place under theshade, in order to prevent cracks, which a more rapiddesiccation would produce. When the rods are dry they® re to be cut into a proper length, and the edges shoulde taken off; after which they must receive the firstcutting, in order to give them a blunt point. The lastoperation is that of scraping them, for the purpose oftaking off the hard, outward coat formed on the surfacem drying, and which would prevent their making anytharks. It may be necessary to rub a small portion of

oxide of iron, 1 ounce; gum arabic, dry, 18 grains : "teehanicalcrayons thus made are very friable, but they may be used , Proc esses 'for large designs. 2. Reddle, &c. 1 ounce, gum 21grains, will yield strong crayons, rather friable, butexcellent for large drawings. 3. Reddle 1 ounce, gum24 grains, or rather 25^ grains, will give smooth andsolid crayons, the best for common use. 4. Reddle 1ounce, gum 27 grains, will produce soft firm crayonsfit for drawings which require delicacy and precision.

5. Reddle 1 ounce, gum 30 grains, will yield very firmcrayons fit for small drawings which are to be highlyfinished. 6. Reddle 1 ounce, gum 33 grains, will givevery hard crayons which cannot be used without someforce. This is the greatest quantity of gum that can beemployed in their composition; with more they wouldbe useless. 7. Reddle I ounce, gum 22 grains, whitehard soap 30 grains. Crayons thus formed have a some-what browner cast than the former. Their consistenceis good, and they may be easily cut. When crayonshave soap in their composition, all the strokes made bythem have always a shining appearance, if the touchesare repeated somewhat too strongly. These crayonsperfectly resemble those of Desmarest. 8. Reddle 1ounce, isinglass 36 grains, produce crayohs of a brilliantcolour and excellent for use. With less isinglass theybecome brittle, with more too hard.

(1136.) Crayons for tracing on glass .—This is an Crayons forinvention of Mr. Howlett for perspective drawings. The tracing oninstrument consists of a plate of clear glass fixed in a S lass>frame, and set upright on a tripod stand so as to bringit on a level with the eye of the artist. From the top ofthe tripod projects horizontally a light frame with a holeat the end for the purpose of supporting in a verticalposition a pin, at the top of which is a small perforation,through which the artist looks while he traces on theglass the objects seen through it. So far the instru-ment does not differ in any material respect from thoseusually employed for this purpose. But as glass is notcapable of receiving the traces made by the pencil, it isnecessary to cover the surface of the glass with somesubstance, at the same time as transparent as possible,and capable of being marked by a pencil. These con-ditions, however, have not hitherto been found to bereconcilable. The most transparent paper that can bemade is not capable of allowing distant objects to beseen through it with sufficient distinctness, and glass

"'‘into the grooves of the wood that the paste may not itself, though unexceptionable with regard to trans-

ndhere too closely to the moulds. Gum arabic and•singlass are the two best substances for mixing with thepowder. The gum and soap may be dissolved in coldwater; but the isinglass must be cut in small pieces,Put into hot water, and dissolved in balneo marise.ese solutions should be well diluted with water thatey may be made to pass through a hair sieve, in order toemove any foreign particles. As it is difficult to in-

parency, will not retain traces from a black-lead pencil,or from any olher of the materials usually employed insketching. The way in which Mr. Howlett solves thisproblem is by the invention of crayons capable of bear-ing a fine point and of leaving traces on the surface ofthe glass. In the use, therefore, of this instrument, theobjects are delineated with the crayon on the surface ofthe glass, and afterwards a piece of paper is laid over

--.. .o —- » .’ u 7s secured "by its four corners to pre-

corporate the paste with the isinglass, they must both be the drawing an rp| ie o-lass with the attached paper

heated and mixed over a fire with a heat equal to that of yent it iroffl 5 >PP a jjTht, and the objects already

boilitlff Wntfln. u jg necessary to mix the paste before it is then held up o » , ’

tmg water: it . - j -- -'TVwith a pestle

is moulded. The best mode is to beat before

or mallet, and so pound it again for som ^ require ait is to be put into the moulds. B " , rp^ e best

but for those crayons in which gum is us • w ^j be tomeans to prevent waste during the opera ; ns;r edi-fix by experiments the quantity of water a . “ forments which the pounded reddle and the » state-

before the mixtures are made. The following^ ^ment will serve this purpose. !■ D r y re ’

“ nn thP o-lass are traced on the paper with a com-drawn on o pape r, instead of being laid on the

Trawinl is placed on the opposite surface of the glass,ffie copy wiU be made in a reversed position, and is thusimmediately adapted to the use of the engraver ; or thetracin' 1 ' on the glass may be transferred to paper, bylaying” the paper over the tracing and rubbing themtogether with an ivory-handled knife. If the tracingpaper employed is thick or opaque, so that the lines on